The importance of crystalline optical fibers goes back at least to the introduction of laser melting technology in 1976. See, e.g., U.S. Pat. No. 3,944,640. Crystalline fibers may be, for example Neodymium (Nd)/Ytterbium (Yb)/Erbium (Er) doped yttrium aluminum garnet (YAG) fibers. In comparison to glass optical fibers, crystalline fibers offer a number of advantages. J. A. Harrington, “Single-crystal fiber optics: a review,” SPIE 8959, p. 895902-1, 2014. For example, crystalline fibers typically have an absorption cross section that is an order of magnitude higher than that of a corresponding glass fiber. This not only reduces the required fiber length for lasing but also significantly mitigates the nonlinear issues.
Crystalline fibers also tend to have a much higher thermal conductivity than glass fiber. For example, a crystalline YAG fiber may have a thermal conductivity of about 10 W/m·K, compared to 1.38 W/m·K for silica fiber. This enables better thermal dissipation by the crystalline fiber. Crystalline fibers (particularly Nd/Yb/Er doped YAG fibers) have a much lower nonlinear stimulated Brillouin scattering (SBS) coefficient than that of silica fiber. This substantially reduces detrimental SBS effect and enables a higher power/energy fiber laser. The ultimate scaling potential for an Yb-doped YAG fiber has been estimated to be as high as 16.9 kW, which is about one order of magnitude higher than that of silica-based fiber laser (˜1.89 kW). J. Dawson, “Power scaling analysis of fiber lasers and amplifiers based on non-silica materials,” SPIE 7686, p. 768611, 2010. Finally, in addition to the potential of enabling higher power/energy fiber lasers and fiber amplifier, crystalline fibers can play an important role in harsh environment high sensitivity and selectivity fiber optic sensors. For example, since the melting temperature of crystalline sapphire fiber is higher than 2000° C., very high temperature (up to 2000° C.) fiber optic temperature sensors may be prepared. S. Yin, P. Ruffin, and F. Yu, Fiber Optic Sensors, CRC Press, New York, 2008. Magneto-optic crystalline fibers [e.g., bismuth substituted yttrium iron garnet (Bi:YIG) crystalline fiber] can also enable high sensitivity fiber optic magneto-optic sensors and all-fiber isolators.
Although crystalline fibers offer a great potential for high power/energy fiber lasers as well as harsh environment fiber optic sensors, the performance of current crystalline fiber based lasers or sensors is largely compromised by a lack of a proper crystalline cladding. A proper cladding can not only reduce the scattering loss but also control the number of modes propagated in the fiber. For many applications, such as high beam quality fiber lasers, fewer or single mode operations are preferred. Unfortunately, unlike glass fiber, crystalline fibers are not pulled from a vitreous melt and therefore cladding cannot be readily fabricated in the same way as amorphous glass fibers. In the past several decades, there have been continuous efforts in developing proper cladding on crystalline fiber cores. Although there has been some progress in this field, high quality crystalline cladding and crystalline core optical fibers are still underdeveloped.